Building structures constructed for human occupancy typically maintain the temperature and humidity conditions inside the building at a comfortable level for its occupants with the use of heating and air conditioning equipment controlled by a thermostat, whereas the temperature outside the building varies with atmospheric conditions. The roof and/or exterior walls of a modern building usually include at least one layer of thermal insulation material to retard the transfer of heat between the inside and outside surfaces of the structure. If the amount of insulation material is sufficient, the transfer of heat during the day from the hot outside portion of the wall or ceiling to the lower inside temperature portion of the structure will be reduced. The air conditioning unit of the building can then compensate for any undesirable increase in temperature. Later, the exterior portion of the wall and ceiling might cool to a temperature that is lower than the inside temperature of the building. In a like manner, the heating unit of a sufficiently insulated building structure compensates for any undesirable decrease in the internal temperature of the building structure.
The rate of heat will flow through a wall or ceiling into or out of a room is dependent upon at least two factors: the temperature gradient between the inside and the outside of the structure, and the efficiency with which the ceiling or wall conducts heat. To reduce the rate of heat transfer across the ceiling or wall into or out of the building structure, a greater quantity of and/or a more efficient insulating material can be used. Such insulating materials include, for example, fiberglass, mineral wood, urethane foams, cellulose, and other materials known in the art.
Though conventional insulating materials can be effective at reducing heat transfer through the walls or ceilings, etc., conventional insulation materials are expensive, bulky, difficult to install, and in some instances are not very effective for specific commercial and private applications. Also, some structures are not built with enough space to accommodate the quantity of insulation materials necessary to adequately insulate the structure.
Heat applied to a phase change material (“PCM”) in a solid state is absorbed by the PCM, resulting in an increase in the temperature of the PCM. As the temperature of the PCM reaches its phase change temperature, i.e., the temperature at which the PCM material changes from a solid state to a liquid state, the PCM stops increasing in temperature and substantially maintains a constant temperature at its phase change temperature, consuming the heat being applied thereto and storing it as latent heat. Latent heat is the heat gained by a substance without any accompanying rise in temperature during a change of state. In essence, it is the amount of heat necessary to change a substance from the solid state to the liquid state. Once the phase change material has completely changed to a liquid state, the temperature of the PCM begins to rise again as the applied heat is now absorbed as sensible heat. In reverse, as the PCM drops in temperature, the latent heat is released at the phase change temperature of the PCM as the PCM changes into its solid state. Examples of PCM's for isothermally storing and releasing heat as described above are paraffin, calcium chloride hexahydrate, sodium carbonate, and Glauber's salt. PCM's have been used in wall structures as described in U.S. Pat. Nos. 5,626,936 and 5,770,295.
A practical issue when using PCM's for isothermally storing and releasing heat in a large structure such as a vertical wall is maintaining the material in a uniform state over a large area. It is desirable that the phase changes occur evenly throughout the PCM, and that there be minimal localized changes that can result in unequal heat distribution. For example, when a PCM liquefies, there is a natural tendency for the liquid to descend under gravity and gather towards the bottom of a vertical wall, or to become absorbed into an adjacent material. Calcium chloride hexahydrate-based PCM, for example, can be made into a super-saturated solution, but when such a PCM mixture goes through the phase change from a solid to a liquid, it will pass through several hydration levels each having a different specific gravity. The liquid mix, as it melts, tends to stratify, with the heavier phases tending to move to the lowest level possible, thereby forming a gradient of layers. A PCM, therefore, should be confined to its intended location and remain in situ, ready for the next (reverse) phase change.
Certain PCM's, when exposed to the atmosphere will either evaporate and dry out or absorb excessive moisture, either event inhibiting or preventing the PCM from functioning as intended to moderate temperature. In particular, PCMs comprising solutions of calcium chloride, if not contained, may be corrosive to metals. Being highly hygroscopic, or deliquescent, if exposed to the atmosphere such PCMs will continue to absorb moisture to the point that that they become dilute aqueous solutions that have lost the desired phase change properties. It is also the case that PCMs may become supercooled and fail to solidify as a result of the lack of nucleation centers.
For these reasons, while the latent heat absorption and release capabilities of certain PCM's have been known and used in limited ways, wide spread practical and commercial use has been hampered.
Confinement or enclosure of the PCM, particularly liquid PCMs such as calcium chloride solutions, is possible in arrays of cells that can be formed between superposed layers of thermoplastic sheets, as taught by U.S. Pat. No. 5,626,936. However, by itself this mechanical means of confining the PCM into multiple small volumes reduces, but does not eliminate the problems of uneven melting and freezing or of leakage of the PCM.
What are needed, therefore, are thermal stabilization compositions that undergo efficient and uniform reversible phase changes. Such compositions should not exhibit phase separation due to stratification or settling, and provide uniform heat transfer over the area of a structure in which the composition in its packaged form is installed.